1. Field of the Invention
The present invention relates to a sintered body for a wire drawing die and a process for the production of the same. More particularly, it is concerned with a diamond compact for a wire drawing die, which contains a binder phase of WC or (Mo, W)C with a small quantity of an iron group metal, and a process for the production of the same.
2. Description of the Prior Art
Up to the present time, WC-base cemented carbides alloys have been used as a tool material for a wire drawing die and natural diamond single crystals have been used for a thin wire, the surface of which should finely be finished. Compacts obtained by sintering diamond powders with binder metals under a superhigh pressure at a high temperature have lately been used as a die and a part of the dies of cemented carbides alloys or natural diamond have thus been replaced by such a die of diamond compact because of its properties excellent in wear resistance as well as strength. The above described diamond compact marketed for use as a die is a sintered body of diamond particles of about 60 microns containing about 10% by volume of a binder phase consisting predominantly of Co. This sintered body has such a structure that the circumference of a diamond sintered body is surrounded by a WC-Co alloy. Japanese Patent Application (OPI) No. 26746/1975 discloses a wire drawing die constructed of such a sintered body, which is produced by filling the inside of a surrounding support consisting of a WC-Co mixed powder compact with diamond powder and subjecting to sintering under a superhigh pressure and at a high temperature. In this case, during the sintering, the liquid phase of the WC-Co alloy penetrates the diamond powder from the circumference thereof, thus forming a binder phase for the diamond sintered body.
When using the diamond sintered body of coarse grain in the field where wire drawing dies of cemented carbides alloys were used in the prior art, the wear resistance has markedly been improved in some instances, but on the other hand, some difficulties have been made clear, one of which is for example that there remain scratches on the surface of a drawn wire as can be seen from the enlarged photograph of FIG. 1.
FIG. 1 is an enlarged photograph showing the surface of a wire when 1.5 tons of a brass-plated steel wire of 0.175 mm in diameter is drawn, and FIG. 2 is an enlarged photograph of the inner surface of a die during the same time. Numerous lengthwise scratches are found on the surface of a drawn wire. These scratches are possible caused by the fact that the binder material of a diamond sintered body used in this die, consisting predominantly of metallic cobalt, tends to adhere to a wire rod to be worked and exhibits a wear resistance considerably different from that of diamond crystal grains, so that the portions of the binder material are first of all worn during wire drawing to form concave portions into which the wire rod to be worked enter.